1. Field of the Invention
The present invention relates to a flux cored wire for welding duplex stainless steel, whose composition ratio of the austenite phase to the ferrite phase is 1:1, and in particular to a flux cored wire, for welding duplex stainless steel, excellent in pitting corrosion resistance, cold toughness and welding usability.
2. Prior Art
Hitherto, duplex stainless steel defined in JIS SUS 329J3L, SUS 329J4L and ASTM S31803 and the like has widely been used as construction materials for chemical plant machines, oil pipes for oil or natural gas drilling, pipelines, chemical tankers, water gates and the like because the steel has excellent stress corrosion crack resistance (SCC) and pitting corrosion resistance mainly in the environment including chlorine ions, such as seawater, and has high strength.
In many cases, for welding these materials, there is basically used a weld metal having the same alloy composition as a parent metal. Therefore, the alloy composition of the weld metal has various kinds in the same way as the parent metal. Concerning welding thereof, various kinds of methods are used. In particular, there is widely used MAG welding by means of a flux cored wire which is easy to use and exhibits high efficency.
However, the duplex stainless steel is subjected to thermal treatment after rolling in the process for producing the steel so that the microstructure of the steel is stabilized to microstructure which is similar to that in the equilibrium state at thermal treatment temperature. On the other hand, the weld metal has relatively stable microstructure in the non-equilibrium state, wherein only the ferrite phase is solidified and subsequently the austenite phase is precipitated in the ferrite phase in the step of natural cooling. For this reason, the pitting corrosion resistance and the notch toughness of the weld metal is more unstable than those of the base material. Thus, there arise problems in some construction materials to which the conventional welding consumable is applied. Thus, an improvement in the welding consumable has been demanded.
In the light of the above-mentioned problems, the present invention has been made. It is therefore an object to provide a flux coredwire, for welding duplex stainless steel, excellent in pitting corrosion resistance, cold toughness and welding usability.
The flux cored wire for welding duplex stainless steel of the present invention, comprises a shell cored with a flux, wherein the total of the shell and the flux comprises C, Cr, Mo and N in amounts of 0.04 wt % or less, 21.0-26.0 wt %, 2.6-4.0 wt % and 0.08-0.30 wt %, respectively, of the total weight of the wire; the amounts of C, Bi and a metal inclusion having a melting point over 2000xc2x0 C. are set up to 0.04 wt % or less, 0.015 wt % or less, and 0.1 wt % or less, respectively; the total amount of Ni powder and Fe powder is set up to 3.0 wt % or less; and a parameter A represented by {Cr}+3.3xc3x97{Mo}+16xc3x97{N}xe2x88x92150xc3x97{Bi} wherein {Cr}, {Mo}, {N} and {Bi} represent the contents of Cr, Mo, N and Bi, respectively, is from 33.0 to 43.0.
In the present invention, it is preferable that the total of the shell and the flux comprises TiO2, SiO2, ZrO2, Al2O3 and MgO in amounts of 5.0-8.0 wt %, 0.2-1.2 wt %, 1.2-3.3 wt %, 0.50 wt % or less, and 0.35 wt % or less, respectively, of the total weight of the wire; a parameter D represented by {Al2O3}xe2x88x9220xc3x97{Bi2O3} wherein {Al2O3} and {Bi2O3} represent the contents of Al2O3 and Bi2O3, respectively, is from 0 to 0.35; and the total amount of the slag forming components is from 9.0 to 13.0 wt %.
In the present invention, it is also preferable that the total of the shell and the flux comprises 1.0 wt % or less of one or more selected from the group consisting of Ti or Fexe2x80x94Ti alloy: 1.0 wt % or less as a value converted to Ti; Mg alloy: 1.0 wt % or less as a value converted to Mg; and Al or Al alloy: 1.0 wt % or less as a value converted to Al.
According to the present invention, the microstructure of the weld metal is made stable and micro-segregation, which is a starting point of pitting corrosion, is reduced by specifying the contents of Cr, Mo and N, which are basic chemical components of a weld material for duplex stainless steel, into such a range that mechanical properties and welding usability of the weld metal are not damaged, specifying the contents of C and Bi in the wire, specifying the parameter A, limiting metal inclusions having a melting point over 2000xc2x0 C. in metals added to the cored flux, and specifying the contents of the Ni powder and the Fe powder. In this way, it is possible to obtain the duplex stainless steel welding flux cored wire which does not damage the pitting corrosion resistance and the welding usability of the weld metal.
The duplex stainless steel welding flux cored wire having still better welding usability can be obtained by limiting the total amount of TiO2, SiO2, ZrO2, Al2O3, MgO and slag forming components and the value of the parameter D appropriately.
The duplex stainless steel welding flux cored wire having still better notch toughness can be obtained by limiting the contents of Ti, Mg or Al appropriately.